Bonding of granular materials to polyolefin surfaces

ABSTRACT

A permanent roughened or textured surface is applied to parts molded from polyolefins, particularly from polyethylene by applying to the surface of the part a coating mixture of a tackifier resin and polyolefin particles in a liquid carrier and incorporating a granular or particulate matter in the coating, either by application to the coating or by admixing the granular or particulate matter into the coating mixture. The coated surfaced is heated to the melt temperature of the polyolefin for a short time, sufficient to gel the polyolefin particles of the coating into the surface of the part, but insufficient to cause any thermal distortion of the part.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] This invention relates to surface treatment of polyolefin parts,in particular, to permanently bonding granular materials to surfaces ofpolyethylene parts such as containers and outdoor signs.

[0003] 2. Brief Statement of the Prior Art

[0004] Polyolefin surfaces, particularly polyethylene surfaces, resistacceptance of coatings. These polymers are commonly used to form large,hollow-form parts such as outdoor signs, tanks and containers byrotational molding. The surfaces of these parts are smooth and difficultto coat.

[0005] In many instances, it is desirable to apply granular orparticulate materials to the surfaces of such parts, e.g., containerswhich also function as steps need a roughened or abrasive texture andoutdoor signs can be enhanced with reflective glass beads ormicrospheres.

[0006] U.S. Pat. No. 5,840,142 discloses and claims a method for thepermanent application of indica to the surface of a molded polyethylenepart using a transfer which is screen printed with a wax, polyethyleneand pigment mixture. After application to the surface of a polyethylenepart, the transfer is coated with a mixture of polyethylene and wax andthe part is heated to fuse the coating and transfer into the surface ofthe part. Unfortunately, the method of this patent cannot be used tocreate a part with a roughened or textured surface, since the methodachieves complete integration of the applied coatings into the surfaceof the part, forming a smooth clear surface.

OBJECTIVES OF THE INVENTION

[0007] It is an objective of this invention to provide a method to applygranular or particulate material to the surface of parts molded ofpolyolefins, particularly of polyethylene.

[0008] It is a further objective of this invention to providepermanently roughened or textured surfaces to parts molded frompolyolefins, particularly from polyethylene.

[0009] It is an additional objective of this invention to provide acoating composition which is useful in the method for applying granularor particulate material to the surface of polyolefins, particularly ofpolyethylene.

[0010] Other and related objectives will be apparent from the followingdescription of the invention.

BRIEF DESCRIPTION OF THE INVENTION

[0011] This invention is a method to impart a permanent, roughened ortextured surface to parts molded from polyolefins, particularly frompolyethylene. The method comprises coating the selected surface of thepart with a coating mixture of a tackifier resin and polyolefinparticles in a liquid carrier and incorporating a granular orparticulate matter in the coating, either by application to the coatingor by admixing the granular or particulate matter into the coatingmixture. The invention also includes the composition of the coatingmixture. The coated surface of the polyethylene part is heated to themelt temperature of the polyolefin for a short time, sufficient to gelthe polyolefin particles of the coating into the surface of the part,but insufficient to cause any thermal distortion of the part. Theheating permanently incorporates the coating into the surface of thepart, with the granular solids bonded to the surface to impart aroughened or textured surface.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0012] The invention is applicable to the treatment of surfaces ofpolyolefins such as polymers and copolymers of ethylene, propylene,butene, isobutene, with minor amounts of other comonomers such as vinylacetate, vinyl chloride, etc. Polyethylene is the most common andadvantageously treated polyolefin and is preferred, however, thesurfaces of other polyolefins can be likewise treated.

[0013] The treatment of the invention is applicable to provide aroughened or textured surface to polyolefins in any form or shape,including films and sheets and molded objects such as containers, e.g.,boxes, tanks, and outdoor signs, which are commonly rotationally moldedfrom polyethylene.

[0014] The method employs a coating adhesive mixture which is applied tothe polyolefin surface as a thin coating approximately several mils orless, preferably one mil, in thickness. The coating can be applied byany conventional method such as spraying, brushing, rolling, etc.Spraying is preferred for ease of application.

[0015] The active ingredients in the coating mixture are powders of apolyolefin, preferable polyethylene and a tackifier. The polyethylenepowder and tackifier are present in relative proportions of 15-30 weightparts tackifier and 85-70 weight parts polyethylene powder per 100weight parts. These ingredients are dispersed in a suitable liquidcarrier to permit application to the polyolefin surface. The liquidcarrier can be water or a hydrocarbon solvent such as hexane or toluene.

[0016] The tackifier should be compatible with the polyolefin andpreferably should be white to neutral in color to avoid staining thepolyolefin surface. It should also have a softening temperature lessthan the melting temperature of the polyolefin, which for polyethyleneshould be less than 250 degrees F. Useful tackifiers include polyacrylicacid polyacrylates, polyurethanes, poly(vinyl)acetate and copolymers andmixtures thereof. Particularly preferred tackifiers are hydrocarbonresins such as aliphatic or cycloaliphatic petroleum resins from fivecarbon monomers containing minor amounts of aromatics, synthetic terpeneresins, chlorinated polyolefins and hydrogenated rosin and rosin esters.The tackifier should be of light color to avoid staining the polyolefinsurface, should have high heat and ultraviolet light stability and besoluble in hydrocarbon solvents or be readily emulsifiable in water. Thesoftening point of the tackifier should be from 75 to 135 degrees F.,preferably from 85 to 120 degrees F.

[0017] The polyolefin ingredient is used in a finely subdivided orpowdered state with a particle size from less than 1 micron to about 140microns, preferably from 5 to about 40 microns, maximum particlediameter. The density of the preferred polyethylene powder ranges fromabout 0.88 to 0.97 grams per cubic centimeter. Low, high and linear highdensity and high to ultrahigh molecular weight polyethylene can be used.Other suitable polyolefins include polypropylene, ethylene/vinyl acetatecopolymers, and metallocene catalyst polyethylene.

[0018] The coating mixture has sufficient liquid carrier to provide asolids content from 30 to about 50, preferably from 25 to 35, weightpercent, and can be thinned with additional liquid carrier to provide anappropriate viscosity for application by spraying, brushing, rolling, ordipping of the polyethylene surface with the coating mixture.

[0019] When water is used as the liquid carrier, the active ingredientsare dispersed in water with surface active agents which can includehydrocarbon silicone and fluorocarbon surfactants, non-ionic surfactantsand ionic surfactants. Sufficient amounts of the surfactant are used toachieve a stable suspension of the ingredients in the aqueous mixture.Typically, the effective concentration of the surfactant will be from0.1 to about 2 weight percent of the solids content in the aqueousmixture, depending on the particular surfactant which is used.

[0020] The granular solids can be of various sources, selected for theparticular application. Examples of suitable materials include silica,e.g., sand, silicon carbide, aluminum oxide, aluminum silicates, garnet,etc. If skid resistance is desired, sand or garnet powders can be used.If an abrasive surface is desired, silicon carbide or alumina can beused. In some applications, a highly reflective surface may be desiredand reflective glass beads or microspheres, or metal flakes can be used.The particle size of the granular solids can be varied considerably from15 to about 300 mesh. The preferred size range is from 100 to about 300mesh.

[0021] The granular solids can be admixed into the coating mixture whenthe mixture is applied by brushing, rolling or dipping. In suchapplications, the coating mixture can be loaded with granular solids inamounts of 5 to 50 weight percent, preferably 15 to 30 weight percent.When the coating mixture is applied by spraying, the granular solids arespread across the wetted surface of the polyethylene part by dusting orother suitable techniques that will ensure uniform distribution of thesolids.

[0022] The coated surface is then heated to raise the temperature of thecoating and outer skin of the surface to the melt temperature of thepolyolefin powders, fusing the powders and tackifier into the outer skinof the polyolefin surface. The granular solids are thereby encased inthe outer skin of the polyolefin part, yet protrude from the surface ofthe part to impart a roughened texture to the surface. In a typicalapplication, the coated polyolefin surface is heated to a temperaturefrom 250 to about 350 degrees F., taking care to avoid excessivetemperatures or temperature differentials which could cause thepolyolefin part to distort or warp.

[0023] The heating can be accomplished using a suitable radiant sourcesuch as an open flame or a high temperature electrical heater, e.g., aninfrared heater. The heating step is practiced to apply heat locally tothe coated polyethylene surface sufficiently to fuse the coating intothe surface of the polyolefin part, a condition which is reached whenthe coated surface appears to be clear of any cloudiness.

[0024] Thereafter, the polyolefin part is cooled to ambient temperature.

[0025] The invention is further described and illustrated in thefollowing example.

EXAMPLE

[0026] A battery container having a cover approximately two by threefeet in area and obtained by rotationally molding from polyolefin istreated by the invention to provide skid resistance suitable for use asa step. A coating mixture of active ingredients comprising 30 weightpercent of an aromatic modified C5 aliphatic hydrocarbon resin and 70weight percent polyethylene powders was used to coat the outer surfaceof the container cover. The polyethylene powder was high densitypolyethylene with a size range from 1 to 80 microns. The activeingredients were dispersed in toluene at a concentration of 25 weightpercent and were sprayed onto the polyethylene surface sufficiently toform a continuous film over the surface. A pumice aggregate containing70 weight percent silica with a size range from 15 to 100 mesh wasdusted over the wet surface of the coating and a second coating of thecoating mixture was sprayed over the surface. The coating was permittedto dry and then the surface was heated to approximately 250 degrees F.,sufficiently to fuse the coating into the outer skin of the polyethylenesurface. After cooling the granular solids were observed to haveimparted a permanent, roughened texture to the polyethylene surface.

[0027] The invention has been described with reference to theillustrated and presently preferred embodiment. It is not intended thatthe invention be unduly limited by this disclosure of the preferredembodiment. Instead, it is intended that the invention be defined by themethod steps and ingredients, and their obvious equivalents, set forthin the following claims.

What is claimed is:
 1. The method of treating a polyolefin object toobtain a permanently textured surface which comprises: a. coating thesurface with a mixture of a tackifier and polyolefin powders in a liquidcarrier; b. incorporating particulate solids having a size range passinga 15 mesh standard screen size into the coating; c. drying the coatingand heating the coating and polyolefin surface to the polyolefin melttemperature for a sufficient time to gel the coating into the surfacebut insufficient to cause thermal distortion of the polyolefin object.2. The method of claim 1 wherein the polyolefin object is a polyethyleneobject.
 3. The method of claim 2 wherein the polyolefin powder ispolyethylene powder.
 4. The method of claim 3 wherein the particles ofthe polyethylene powder have a size range less than 140 microns.
 5. Themethod of claim 1 wherein said tackifier is an aliphatic orcycloaliphatic hydrocarbon resin.
 6. The method of claim 1 wherein saidactive ingredients comprise from 15 to 30 weight percent tackifier andfrom 85 to 70 weight percent polyolefin powder.
 7. The method of claim 6wherein said liquid carrier is a hydrocarbon solvent.
 8. The method ofclaim 6 wherein said liquid carrier is water and including sufficientsurfactant to disperse the active ingredients in water.
 9. A coatingmixture comprising: a. active ingredients comprising from 15 to 30weight percent tackifier and from 85 to 70 weight percent polyolefinpowders having a size range from 1 to about 140 microns; and b. a liquidcarrier selected from the group consisting of hydrocarbon solvents andsurfactant containing water in an amount from 1 to about 2 weight partsper part of active ingredients.
 10. The coating mixture of claim 9wherein the polyolefin powder is polyethylene powder.
 11. The coatingmixture of claim 10 wherein the particles of the polyethylene powderhave a size range from 5 to about 40 microns.
 12. The coating mixture ofclaim 1 wherein said tackifier is an aliphatic or cycloaliphatichydrocarbon resin.